Hydrogels and their use as contact lenses have been known since at least Wichterle et. al. published U.S. Pat. No. 3,220,960, which discloses sparingly crosslinked, hydrated poly(hydroxyalkyl methacrylate), typified by poly(2-hydroxyethyl methacrylate) (poly-HEMA) with a water content of about 39%.
Poly-HEMA became the standard material for hydrogel contact lenses since it is hard enough to be easily fabricated by machining and polishing in the dry state, yet soft and comfortable to wear in the water swollen state.
In subsequent developments other hydrophilic monomers were used, most commonly N-vinyl-pyrrolidone (NVP) copolymers with methyl methacrylate (MMA) or other "high Tg" methacrylates. With this system, hydrogels with water contents up to 80% can be prepared. Dimethylacrylamide copolymers provide similar properties and have been described as well.
The oxygen transmissibility of such conventional hydrogel contact lenses is determined by their water content and thickness and can be improved by increasing the water content or by decreasing thickness. Both strategies have been used to increase O.sub.2 -permeability and to make extended-wear contact lenses, but both strategies lead to lenses with insufficient strength which are easily damaged. It is highly desirable to have a hydrogel soft lens with the same or similar good mechanical properties and as comfortable to wear as poly-HEMA, yet with substantially higher oxygen permeability. This can now be achieved by incorporation of either siloxane groups or fluorinated groups into the polymer compositions.
It would be especially desirable to have highly fluorinated hydrogels since, while siloxane groups give slightly higher oxygen permeability, fluorinated groups allow the manufacture of polymers with higher dry hardness and therefore better machinability while at the same time reducing lipophilicity and deposit formation on the hydrated polymer.
Among prior art compositions consisting of fluorinated hydrogels the following patents are relevant:
U.S. Pat. Nos. 4,433,111 and 4,493,910 describe hydrogels and contact lenses obtained by copolymerization of 20-40 mol % substituted or unsubstituted acrylamide or methacrylamide; 25-55 mol % N-vinylpyrrolidone (NVP); 5-20% mol hydroxy-alkyl(meth)-acrylate; 1-10 mol % (meth)-acrylic acid, and 1-9 mol % of a perfluoroalkyl-alkylene(meth)-acrylate; the perfluoroalkyl groups act to reduce protein deposition.
U.S. Pat. No. 4,640,965 describes hydrogels and contact lenses obtained by copolymerization of hydroxyfluoroalkylstyrene (5-60%, by weight), with hydroxyalkyl (meth)-acrylates or N-vinylpyrrolidone (40-95%, by weight); the hydroxy group is necessary to attain the required compatibility.
U.S. Pat. No. 4,638,040 describes the synthesis of 1,3-bis(trifluoro-acetoxy)propyl-2-methacrylate polymers and their use as hydrogel-contact lens materials and ocular implants after hydrolysis.
U.S. Pat. No. 4,650,843 describes hydrogel contact lens materials consisting essentially of copolymers of 50-95% (by weight) of 2-hydroxyethyl-methacrylate and 5-35% (by weight) of fluorinated methacrylates with up to 5 F-atoms.
In all these cases the range of clear compositions is very limited; the commercially available fluorinated (meth)acrylates can be incorporated in only relatively small amounts; alternatively, complicated, for instance hydroxylated F-monomers have to specially be synthesized to achieve better solubility in NVP or HEMA (U.S. Pat. No. 4,640,965). It has now unexpectedly been discovered that N,N-dimethylacrylamide when used as comonomer with fluorine-containing monomers gives clear copolymers within a wide range of possible compositions. This has been especially surprising since N-vinyl pyrrolidone (NVP), which has a solubility parameter, polarity and hydrogen-bonding capacity very similar to dimethylacrylamide (DMA), does not give clear compatible mixtures under the same conditions. [The solubility parameters (cal/cm.sup.3).sup.1/2 for the analogous saturated molecules are: N,N-dimethylacetamide, C.sub.4 H.sub.9 ON: 10.8, moderate H-bonding; N-methylpyrrolidone, C.sub.5 H.sub.9 ON; 11.3, also moderately H-bonding.] In addition, the Copolymerization between DMA and acrylates and methacrylates in general proceeds much smoother because of more favorable reactivity-ratios, leading to a more random copolymer structure for DMA-copolymers than for NVP-copolymers. This, together with the good compatibility of DMA with fluorinated (meth)acrylates allows synthesis of highly O.sub.2 -permeable hydrogels which are harder than the corresponding silicone-hydrogel copolymers and, because of the oleophobic nature of fluorinated groups, more resistant to soiling and deposit formation.
Crosslinked dimethylacrylamide copolymers with other acrylic or methacrylic monomers and their use as conventional hydrogel-soft contact lenses are described in U.S. Pat. Nos. 4,328,148, 4,328,428 and 4,388,436.
Among silicone containing hydrogels of the prior art, U.S. Pat. Nos. 4,139,692 and 4,139,513 specify tri-siloxy-hydroxyalkylmethacrylate, with the OH-group required for compatibility; DMA is not exemplified, but is claimed together with HEMA and NVP.
U.S. Pat. Nos.4,182,822 and 4,343,927 claim C.sub.1 -C.sub.4 -dialkylacrylamide hydrogel-copolymers with oligosiloxanylsilyl-alkylene methacrylates, but without examplifying DMA copolymers.
Dimethylacrylamide (DMA) has not been used prior to this invention as the major hydrophilic monomer in silicone- and/or fluorine containing hydrogels, linear, not crosslinked copolymers of 2,2,2-trifluoroethyl methacrylate and N,N-diemthyl acrylamide are described in JP 62-115009 as a clear condensation preventing film.
It has also been discovered that DMA-copolymers with fluorinated acrylates or methacrylates with at least 5 fluorine atoms in the ester group, if prepared in the absence of a crosslinking agent, form linear clear polymers which are plasticized, but not dissolved in water. They can therefore in their water plasticized state (hydroplastic) easily be molded, coated or formed into shapes and subsequently crosslinked. This represents another practical method for manufacturing hydrogel articles.